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Excitation of the 3P0 state is detected in optical-optical double resonance via the dark periods in the fluorescence on the fast 1S0 3P1 transition. The dye laser formerly used to excite this line was recently replaced by a diode-based laser system: a grating stabilized diode at 922 nm, further frequency stabilized onto a Fabry-Perot interferometer, is seeded into a tapered amplifier. Subsequent frequency-quadrupling employing a periodically poled KTP and a BBO crystal, each placed in an external enhancement cavity, leads to the right frequency at 231 nm. For the 1S0 3P0 transition a resolution of 1.3 × 10-13 has been achieved so far, limited by the frequency instability of the Nd:YAG laser. The short-term frequency stability of this laser has been strongly improved recently. With a new laser setup a linewidth < 5 Hz (FWHM) for integration times ×30 s has been achieved using active vibration isolation platforms for the reference cavity. Recent progress of that work is reported.